1. Field of the Invention
The present invention relates to an optical module, transmitter and WDM transmitting device that are used in the wavelength division multiplexing (WDM) communication system. In the field of dense WDM, it is generally required that the optical signals are stable in wavelength for a long time period. Thus, a technique of causing the optical module to have a wavelength monitoring function has been developed.
2. Discussion of the Background
One of the prior art documents that discloses an optical module having a wavelength monitoring function is Japanese Patent Laid-Open Application No. Hei 12-56185. Referring first to FIG. 20, there is shown an optical module constructed according to the prior art. As shown in FIG. 20, the optical module includes a light-emitting device 50 including a semiconductor laser diode or the like for outputting a laser beam with a predetermined wavelength; an optical fiber 51 optically coupled with the light-emitting device 50 and adapted to externally deliver the laser beam output from the light-emitting device 50 at its front facet (right side as viewed in FIG. 20); an optical filter 52 having a cutoff wavelength substantially equal to the lasing wavelength of the light-emitting device 50; a beam splitter 53 including a half mirror for dividing a monitoring laser beam output from the light-emitting device 50 at its back facet (left side as viewed in FIG. 20) into two laser beam components; a first photo detector 54 including a photodiode or the like for receiving one of the two laser beam components divided by the beam splitter 53 after it has passed through the optical filter 52; a second photo detector 55 including a photodiode or the like for receiving the other laser beam component from the beam splitter 53; and a Peltier module 56 for regulating the temperature in the light-emitting device 50. A control unit 57 is connected to this optical module and adapted to control the Peltier module 50 to control the wavelength in the light-emitting device 50, based on PD currents outputted from the first and second photo detectors 54, 55.
FIG. 21 is a block diagram of a layout relating to the control unit 57. As shown in FIG. 21, the control unit 57 may have a first transformer 67 for transforming a first PD current outputted from the first photo detector 54 into a first voltage V1, a second transformer 68 for transforming a second PC current outputted from the second photo detector 55 into a second voltage V2, a comparator 69 for comparing the first voltage V1 from the first transformer 67 with the second voltage V2 from the second transformer 68 and for outputting the difference between these two voltages as a control signal, and a thermo electric cooler (TEC) type current generator 70 for outputting a temperature control current used to raise or lower the temperature in the Peltier module 56.
Between the light-emitting device 50 and the optical fiber 51 is disposed a condensing lens 58 for coupling the laser beam from the front facet of the light-emitting device 50 with the optical fiber 51. Between the light-emitting device 50 and the beam splitter 53 is disposed a collimating lens 59 for collimating the laser beam outputted from the back facet of the light-emitting device 50.
The light-emitting device 50, condensing lens 58 and collimating lens 59 are fixedly mounted on a LD carrier 60. The first and second photo detectors 54, 55 are fixedly mounted on first and second PD carriers 61, 62, respectively.
The beam splitter 53, optical filter 52 and first and second PD carriers 61, 62 are fixedly mounted on a metallic base 63, which is in turn fixedly mounted on the surface of the LD carrier 60. This LD carrier 60 is fixedly mounted on the Peltier module 56.
The light-emitting device 50, beam splitter 53, optical filter 52, condensing lens 58, collimating lens 59, LD carrier 60, first PD carrier 61, second PD carrier 62, metallic base 63 and Peltier module 56 are housed within a package 64. A ferrule 65 for holding the tip end of the optical fiber 51 is fixedly mounted on the package 64 at one side through a sleeve 66.
The laser beam that is output from the front facet of the light-emitting device 50 is condensed by the condensing lens 58 into the optical fiber 51 held by the ferrule 65 and externally delivered therefrom.
On the other hand, the laser beam outputted from the back facet of the light-emitting device 50 is collimated by the collimating lens 59 and then divided by the beam splitter 53 into two laser beam components, one being directed in the direction of Z-axis (or transmission) and the other being directed in the direction of X-axis perpendicular to the direction of Z-axis (or reflection). The laser beam component directed in the direction of Z-axis is received by the first photo detector 54 while the laser beam component directed in the direction of X-axis is received by the second photo detector 55.
PD currents outputted from the first and second photo detectors 54, 55 are fed into the control unit 57 which in turn controls the temperature in the Peltier module 56 to control the wavelength in the light-emitting device 50, based on the inputted PD currents.
FIG. 22 is a graph illustrating the age degradation of a laser diode. As shown in FIG. 22, the threshold value in the optical module including the laser diode is Ith when it is initially driven. An auto power control (APC) circuit for driving the optical module to provide a predetermined optical output Pf is provided.
When the optical module is initially driven, a current injected into the laser diode to provide the optical output Pf is Iop. As the laser diode is used for a prolonged time period, its characteristic will be degraded. Thus, the threshold value on termination of a predetermined time period will be raised to Ith′. Moreover, the injection current into the laser diode will be raised to Iop′.
As shown in FIG. 23, the lasing wavelength in the laser diode has a dependency on injection current if the temperature in the LD carrier (sub-mount) is constant. This dependency is usually at about 0.01 nm/mA. Therefore, the lasing wavelength will be shifted toward longer wavelength if the temperature at the LD carrier is constant and when the age degradation in the laser diode occurs.
The optical filter is used for locking the wavelength in the laser diode having such a characteristic. In other words, the temperature in the LD carrier on which the laser diode is placed is regulated by the Peltier module while monitoring the wavelength. The lasing wavelength in the optical module is then fixed to such a wavelength locking point P as shown in FIG. 24. As the injection current increases due to the age degradation of the laser diode, the temperature in the laser diode at its active layer will increase and cause a shift in the lasing wavelength toward longer wavelengths. However, as will be discussed more fully below, the wavelength shift can be compensated by driving the wavelength monitor using the optical filter. Moreover, temperature-dependent changes in the optical filter characteristic can be compensated for by the controller, which in turn controls the cooling level imparted by the Peltier module on all of the components mounted to it. Thus, the temperature in the LD carrier can be lowered by the Peltier module, as can the operational characteristics of the optical filter.
Now, the optical filter is formed, for example, from fused silica. This means that it has a temperature dependency relating to its light transmission (which will be simply referred to “temperature characteristic”). For example, an optical filter may have its characteristic of wavelength-light transmission which is shifted toward shorter wavelength at a rate of 0.01 nm/° C.
In the optical module of the prior art, the light-emitting device 50 may thermally be coupled with the optical filter 52 to maintain substantially the same temperature therein, as shown in FIG. 20. Thus, the temperature in the optical filter 52 will decrease as the temperature in the LD carrier 60 on which the light-emitting device 50 is mounted decreases. Thus, the characteristic in the optical filter 52 will also be changed. In other words, as the performance of the light-emitting device 50 is degraded with time during a predetermined time period after the wavelength monitor has been driven, the injection current into the light-emitting device 50 will increase so as to produce a constant output power, but this increase in injection current also raises the temperature therein. To compensate the wavelength thus deviated, the control unit 57 is driven to control the Peltier module 56 to lower the temperature in the light-emitting device 50, although when changed the temperature in the optical filter 52 will decrease as well. When the temperature in the optical filter decreases, the initial wavelength characteristic will not be provided.
As shown in FIG. 25, thus, the optical filter characteristic will wholly be shifted toward shorter wavelengths. In FIG. 25, black circles indicate initial locked wavelengths P and white circles denote locked wavelengths after the LD driven for a predetermined time period. Thus, the present inventors recognized that the conventional LD module according to the prior art could not provide a laser beam having its desired wavelength since the locked wavelength has been shifted from P to P′. The present inventors further recognized that the relationship between the injection current and the locked wavelength when the wavelength monitor is driven is shown in FIG. 26, showing that the lasing wavelength has a current dependency.
Even when the Peltier module 56 on which the optical filter is mounted is controlled to have its temperature constant, the temperature in the optical module will be varied depending on the change in the external ambient temperature and power consumption in the optical module. Thus, the characteristic performance of the optical filter will be directly influenced by the change in the current temperature through the side thereof which is not in direct contact with the Peltier module. For example, the present inventors observed that the temperature in the optical filter may vary as shown in FIG. 28.
Such a deviation associated with the change of temperature in the optical filter causes the degradation of signal quality through crosstalk and is undesirable for dense WDM systems that require stable wavelengths to operate efficiently and reliably.
Since dense WDM systems use a narrow spacing between optical signal wavelengths, it is under a severe requirement for prevention of the deviation in the wavelength of the respective optical signals. Therefore, higher quality WDM systems use fixed lasing wavelengths, which ensure increased accuracy and signal separation. For example, if optical signals are to be arrayed using an etalon filter having such a wavelength discrimination characteristic as shown in FIG. 27 as an optical filter, the etalon filter must be configured to have a slope having its central or near point overlapped on a predetermined wavelength so that the optical signals are arrayed with a fixed spacing of wavelength. The characteristics of etalon filters are described in section 4 of Yariv, A., “Optical Electronics in Modern Communication,” fifth edition, Oxford University Press, Inc., 1997, the entire contents of which being incorporated herein by reference.
European Patent Application EP1069658 describes a technique of sensing the temperature of an etalon filter and feeds a correction signal from a correction unit to a control unit to compensate the temperature. Generally, the etalon filter has a temperature characteristic. A material used for forming the etalon and having its smaller temperature characteristic is crystal. The crystal has been used even in the aforementioned European Patent Application EP 1069658. The temperature characteristic in the crystal etalon is known to be 5 pm/° C.
The casing temperature in the package used for the optical module must range between −5 and 70° C. Thus, the drift due to the temperature of the etalon filter becomes 5 pm/° ×75° C.=375 pm.
When the temperature in a temperature regulator on which the optical filter is mounted is changed, the drift due to the variation of temperature in the etalon filter will further be increased.
FIG. 29 shows the relationship between the locked wavelength and the locking point on slope if a crystal etalon having a spacing of 100 GHz (800 pm) is used to lock the wavelength and to perform the temperature compensation. The temperature compensation enables the locked wavelength and the locking point on slope to be active on slope.
On the other hand, the field of WDM and particularly dense WDM requires a great number of laser modules having different light-emitting wavelengths. It is not realistic to produce all of such laser modules with their different specifications. It is desirable that one laser module can be regulated to accommodate itself to several necessary wavelengths or at least two wavelengths. To enable such a regulation of wavelength, the effective material for the optical filter used in the wavelength monitor is the etalon that has a repeated cycle of wavelength transmission relative to the wavelength of the necessary laser beam.
However, it is impossible that a wavelength in the repeated cycle of wavelength transmission on the optical filter on which the light-emitting wavelength of the laser is positioned is judged from the signal from the wavelength monitor.
To make it possible, it is required that the laser light-emitting wavelength is controlled to be within a predetermined range of wavelength which can be pre-regulated by the wavelength monitor. When it is wanted to control the light-emitting wavelength of a light-emitting device through a temperature regulator on which the light-emitting device is mounted, the temperature in the light-emitting device must accurately be measured and controlled. It is thus required to place a temperature-sensing unit adjacent to the light-emitting device.
The temperature around the light-emitting device varies depending on the injection current into the light-emitting device or the like. There is also a temperature distribution since the optical filter is spatially spaced apart from the light-emitting device even though they are within the same package and on the same temperature regulator. It is thus difficult that the temperature of the optical filter is compensated based on the result measured by the same temperature-sensing unit.
It is now assumed that the lock point is in the center of the slope at the intermediate temperature, 32.5° C. In such a case, the temperature of the etalon is on a point in the lower and gentler section of the slope at −5° C. and on the maximum value of the photo detector at 70° C. The wavelength locking detects which side of the slope the wavelength drifts on. Therefore, the locking will not sufficiently be performed on the illustrated lower and higher temperature sides. Particularly, the lock point will move to the adjacent slope beyond the peak of the wavelength discrimination. It is thus impossible that the wavelength locking is performed by executing the temperature compensation of a short-cycle etalon filter used in such a dense WDM system. If the spacing of wavelength is gradually reduced to 50 GHz, 25 GHz, 12.5 GHz and etc. to improve the capacity of transmission, the range in which the locking can be made apparently become narrower than the range of temperature compensation, 345 pm. The wavelength locking cannot further be performed.
For such a reason, the dense WDM system having its reduced spacing of wavelength must suppress the wavelength drift within several pm. The conventional optical modules and transmitters could not fulfill the aforementioned requirements since they had had 10 pm only on the casing temperature dependency.
Since the optical module is temperature controlled only through the bottom thereof, each of the parts thereof will have a temperature distribution. Particularly, the etalon filter must have its magnitude equal to or larger than 1 mm since the characteristic of transmission wavelength is determined by the length of the filter along the optical axis and since the filter must have its incident area equal to or larger than the diameter of the incident beam.
In the crystal etalon filter which has its thermal conductivity smaller than those of the metals, the thermal conductivity along the optical axis is equal to 0.0255 Cal/cm-sec-deg while the thermal conductivity along a direction perpendicular to the optical axis, that is, a direction perpendicular to the regulation face of the temperature regulator is smaller, 0.0148 Cal/cm-sec-deg. This makes the control of the temperature regulator difficult and tends to create a temperature distribution in comparison with the other parts such as the light-emitting device and so on.